The present invention relates to olefin copolymers, such as ethylene copolymers, containing a hydrolyzable component that allows the copolymer to be broken down into dispersable fragments upon exposure to aqueous conditions. The copolymers are prepared by transition metal-catalyzed polymerization.
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Over the past several years, a great deal of research has been directed to the design of biodegradable polymers. Concern over waste disposal, particularly of packaging materials, disposable diapers, etc., motivates much of this work. Products designed to degrade after their intended use may be composed of photosensitive or hydrolytically degradable polymers. Polymers intended to degrade during use, as in controlled-release delivery systems, are nearly always hydrolytically degradable. Hydrolytically degradable polymers are frequently polyesterbased, prepared by condensation or radical polymerization. Also employed are physically or chemically bonded blends of synthetic polymers with biomaterials such as starch.
For many purposes, the superior physical properties provided by polyolefins prepared by addition polymerization are desirable. To date, however, incorporation of polar groups into such polyolefins has had limited success, since many polar monomers poison, or competitively coordinate with, the organometallic polymerization catalysts that are typically used. Copolymers of olefins, such as ethylene, with polar monomers such as acrylates, were initially limited to block copolymers, formed by two-stage polymerization, e.g., by post-polymerization of an acrylate or methacrylate monomer onto a previously formed polyolefin chain (Yasuda et al., 1996, 1997; Goto et al., Hajime et al.). Masakazu et al., in J P Kokai 4-45108 (1992), described the preparation of an ethylene copolymer containing 4.7 mole % ethyl acrylate, Mn 9,100, Mw 22,500, that show improved adhesion over polyethylene homopolymer. Johnson et al. (1996) described the formation of random olefin-acrylate copolymers using Brookhart-type catalysts. None of these polymers, however, include a hydrolyzable linkage in the backbone of the polymer, and therefore they would not be hydrolytically degradable. Ouchi et al. described free radical copolymerization of diallylidene pentaerythritol with styrene; however, incorporation of the diene monomer was low, and increased incorporation significantly decreased the intrinsic viscosity of the product. The reaction conditions would also be expected to produce a non-stereoregular polymer.
Accordingly, providing olefin copolymers that have good physical properties and stability under conditions of use, at neutral or near-neutral pH, but that degrade into soluble or easily dispersable particles in an aqueous medium would be desirable.
In one aspect, the present invention is directed to a degradable olefin copolymer. The polymer backbone comprises at least two monomer units. The first monomer unit is non-hydrolyzable and of the form xe2x80x94R1CHxe2x80x94CHR2xe2x80x94, in which R1 is hydrido or alkyl and R2 is hydrido, alkyl, alkenyl, aryl, alkaryl or halogen or in which R1 and R2 are linked to form xe2x80x94Qxe2x80x94 in which Q is substituted or unsubstituted hydrocarbylene. The second monomer unit is hydrolyzable and of the form xe2x80x94CHR3xe2x80x94CHxe2x80x94(L1)mxe2x80x94Xxe2x80x94(L2)nxe2x80x94CHxe2x80x94CHR4xe2x80x94 in which R3 and R4 are independently hydrido or alkyl, L1 and L2 are optionally substituted hydrocarbylene groups, m and n are independently 0 or 1, and X is a group that is hydrolytically cleavable. Preferably, X does not include a hydroxyl group, a primary or secondary amino group, a thiol group, or a group effective to oxidize a metal center of the soluble transition metal catalyst. Examples of suitable X moieties include, but are not limited to, an acetal linkage, a ketal linkage, an ester linkage, and an imide linkage. It is also preferred that there are no adjacent hydrolyzable monomer units in the copolymer. The mole percent of the second monomer unit in the copolymer generally ranges from about 0.1 mole percent to about 50 mole percent.
In a related aspect, the invention provides a method for preparing a degradable olefinic copolymer by addition polymerization, in the presence of a suitable transition metal catalyst and a suitable catalyst activator, of a first olefinic monomer of the form CHR1xe2x95x90CHR2, wherein R1 and R2 are as defined above, with a second olefinic monomer of the form CHR3xe2x95x90CHxe2x80x94(L1)mxe2x80x94Xxe2x80x94(L2)nxe2x80x94CHxe2x95x90CHR4 wherein R3, R4, L1, L2, m, n and X are as defined above.
The transition metal catalyst may be a metallocene complex of a Group IV, Group V, or Group VI transition metal, such as Ti, Hf, Zr, V, Nb, or Mo. Other effective transition metal catalysts are imine, preferably diimine, complexes of Group I or Group VIII transition metals, such as Pd, Ni, Fe, Co, Cu, Ag, and Au, and imine-containing ligands.
These and other objects and features of the invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.